1. Field of the Invention.
The invention relates to servo systems, and more particularly to a servo system which compensates for lateral deviations of a transducer from a data path in a recording medium.
2. Prior Art.
In data recording media, it is customary to record data in formats wherein waveforms representing data are disposed in parallel paths. To reproduce the data, a transducer is used to convert the recorded waveforms into electrical signals. The transducer must accurately follow the center of the data paths in order to obtain the best reproduction of the recorded waveforms. Previously, servo systems have been used to maintain the transducer in a proper path following position, near the path center line as measured from side to side. A common prior art servo system relies on recording of servo waveforms in tracks on a separate servo medium which is maintained parallel to other media, usually in a stacked relationship having user data recorded in paths which correspond to the servo tracks. Typically the paths overlie one another, forming a cylinder. A transducer accurately positioned over a servo track is ganged with transducers over the data paths for very fine alignment of the latter. The paths and tracks are equivalent, but tracks refer to positions for servo data, while paths refer to position for user data.
It has been previously recognized that there are a number of advantages in combining servo waveforms and user data waveforms on the same medium. This is especially important in small systems. For example, if only one user data medium is required, a separate servo medium parallel to the user medium would mean that one half of the total media space is dedicated to servo waveforms, an unacceptably high level. On the other hand, if servo and user data could be combined, a separate servo medium would not be needed.
Momentarily disregarding the problem of where to put servo waveforms, most formats for representing servo data used in fine positioning of a transducer over a data path can be classified as: (1) phase discriminating, (2) limited pulse types and (3) dual frequency. In the first case, two servo waveforms having a known phase relationship are recorded on adjacent servo tracks and are simultaneously read and compared to control transducer position. For example, see U.S. Patent 3,427,606 to Black and Sordello. In the second case, positive and negative pulses are recorded on adjacent servo tracks with periodic polarity reversals. Polarity reversals in two adjacent tracks are sensed by a transducer and amplitudes are compared for centering the transducer. For example, see U.S. Pat. No. 3,534,344 to Santana and U.S. Pat. No. 3,691,543 to Mueller. In the third case, two waveforms representing two different frequencies are recorded on adjacent servo tracks with a servo signal being derived by separation of the frequencies and comparison of the amplitudes of the two signals. For example, see U.S. Pat. No.3,864,740 to Sordello and Cuda.
One of the problems with the first format is that unwanted phase shifts due to speed variations of the media or recording arm head sway or anomalies in the electronics for reading or writing data can cause phase errors, creating uncertainties in the servo information. In the second format, media defects, such as small holes or anomalies in the media or particles of the media surfaces can block detection of pulses, also creating errors. For these reasons and others the third format has been adopted herein.
Returning now to the problem of where to place the servo information, U.S. Pat. No. 3,864,741 to Schwartz, using a modification of the third format, the dual frequency approach, teaches that circumferentially spaced sectors interrupting user data paths allow servo information to be placed on the same medium with user data without requiring the expense of making two different media layers on the same disk, one for user data and one for servo data. However, the aforementioned patent uses two frequencies which must be widely spaced for unambiguous detection and equalization of the amplitudes of the two signals.
The aforementioned U.S. Pat. No. 3,864,740 to Sordello and Cuda teaches the advantage of using two closely spaced servo frequencies, as opposed to the previously mentioned widely spaced frequencies, for correcting transducer position. In review, certain advantages accrue in the use of such closely spaced servo frequencies because (1) the recording characteristics of the transducer change with frequency and therefore the magnitude of the signal from the recorded tracks varies with frequency; (2) the flying height of the transducer varies and as a result the two signals are attenuated at rates varying with frequency; (3) the readback characteristics of the read/write head are different for the two frequencies; (4) the magnetic characteristics of the recording medium change with frequency; (5) the electronic characteristics of the circuit may change with frequency; and (6) changes in the relative speed between the recording medium and the transducer can alter the frequency of the readout signal sufficiently to detune the electronic filters which are frequency dependent.
Sordello and Cuda relied upon a modulation technique to reduce their two closely spaced servo frequencies to two low frequencies suitable for controlling an actuator. The prior art Sordello and Cuda modulation technique used a phase locked oscillator to generate modulating signals to be combined with the closely spaced servo frequencies. However, a new problem now arises.
The phase locked oscillator requires a synchronizing reference frequency. Of course, such a synchronizing reference can be provided by a separate source such as a clock track or even by a separate glass disk or timing gear rotating with the medium and having marks which initiate timing pulses at a desired rate. However, these sync reference frequency sources introduce additional tolerances, components and space requirements which are to be avoided where only a single or a few stacked recording media are to be used.